1. Field of the Invention
This invention relates to an ionization detector using an alternating current (AC) discharge through a working gas as an ionization source for detection of ionizable gases.
2. Description of Related Art
The monitoring of the concentrations of volatile organic compounds (VOCs) and inorganic gases such as ammonia, phosphine, nitrogen oxides, and halogens is important in industrial applications. For such monitoring, a number of gas detectors have been developed.
A photo-ionization detector (PID) can monitor ionizable gases in air without separating the gases. The ionization potentials (IPs) of most VOCs and some inorganic gases are lower than the ionization potentials of the common components of air. For example, nitrogen, oxygen, water vapor, and carbon dioxide, which are the most common components of air, have ionization potentials of 15.58, 12.07, 12.61, and 13.77 eV, respectively, and the IPs of most of the VOCs are less than 12.0 eV. Accordingly, a PID can use an ionization source having maximum photon energy between 12 eV and the IP of a target gas to detect the target gas without ionizing the common components of air.
A common PID uses a sealed vacuum or low pressure lamp to generate UV photons. The UV photons pass through an optical window into an ionization chamber and ionize molecules having ionization potentials below the maximum photon energy of the lamp. After the ionization, one or more measuring electrodes of the PID measure the ion current that the ionized gas molecules cause. The measured current indicates the concentration of ionizable molecules in ionization chamber. One significant disadvantage of PIDs is the relatively short life of the UV lamp caused by window material deterioration, gas leakage into or from the lamp, and window surface contamination.
A flame ionization detector (FID) uses a chemical flame as an ionization source. A typical FID burns a working gas such as hydrogen in air. FIDs have several disadvantages that prevent their use as portable gas monitors. In particular, a FID requires a hydrogen source that must be replenished after a relatively short period of use. Typically, maintaining an adequate flame requires a minimum flow rate of about 15 ml/min of hydrogen. At that flow rate, hydrogen in a 75 ml cylinder with pressure of 1,800 psi can only sustain the flame for about 8 hours. Further, hydrogen presents the danger of an explosion. Also, the hydrogen-oxygen flame is a natural flame that has a directional effect on both baseline and span signals of the FID.
A discharge ionization detector (DID) is another device for measuring levels of VOCs and inorganic gases. A DID applies a high voltage across discharge electrodes to create an electric discharge or spark in a discharge chamber filled with an inert gas such as helium. The electric discharge through the inert gas creates active species such as UV photons and excited atoms. The active species that enter an ionization chamber containing a sample gas ionize components of the sample gas.
U.S. Pat. Nos. 4,028,617; 4,266,196; 4,789,783; and 4,975,648 which are incorporated here by reference in their entireties, describe DIDs having a DC discharge as ionization source. Papers by Jin et al. (J. Chromatograph. A, 761, p. 169, 1996, and Microchem. J., 52, p. 139, 1995), which are incorporated here by reference in their entireties, also describe DIDs using DC discharge. The known DIDs have problems with anode sputtering and contaminant deposition on the surfaces of discharge electrodes. The sputtering and deposition change the discharge electrodes and reduce reproducibility and long term stability of the DID.
U.S. Pat. Nos. 5,153,519; 5,317,271; 5,394,090; 5,394,091; 5,394,092; and 5,541,519, which are incorporated here by reference in their entireties, disclose pulsed DC DIDs. Pulsed DC discharge, in which the discharge is intermittent, can reduce the anode sputtering. However, the direction of the discharge electric field is constant, and anode sputtering and cathode contamination remain concerns.
U.S. Pat. No. 5,086,254, which is incorporated here by reference in its entirety, describes the application of a microwave induced plasma as an ionization source. A paper by Jin et al. (Microchem. J., 35(3), p281, 1987), which is incorporated here by reference in its entirety, introduces a surfatron-based microwave plasma as an ionization source for an ionization detector. The direction of the microwave electric field alternates, which solves the problems of anode sputtering. However, this detector requires a microwave generator and a special microwave transmission line, which make the detector more complicated and less portable. Further, microwave systems require a relatively large amount of power and have not been used in portable detectors.
The above-described DIDs and microwave-driven detectors are conventionally detectors for gas chromatography and have not been used for real-time gas monitoring. Most conventional DIDs use helium as a discharge gas. Basically, UV photons generated by an electric discharge through helium are a universal ionization source for gas chromatography detection because helium has high IP (24.58 eV). Photons from helium typically ionize all species of molecules in the sample gas. This is fine for gas chromatography because a gas chromatographic column separates the different constituents of a gas sample for separate ion current measurements. However, electric discharge in helium is not a suitable ionization source when monitoring VOCs and other contaminants in air because both the contaminants and the components of air are ionized.
U.S. Pat. Nos. 4,447,728 and 5,192,865, which are incorporated here by reference in their entireties, respectively disclose a discharge ionizer for mass spectrometry and an inductively coupled discharge combined with mass spectrometry. However, these systems are for fixed mass spectrometric systems, not for DIDs.
U.S. Pat. No. 4,609,875, which is incorporated here by reference in its entirety, discloses a real-time halogen gas detector that measures the variation of discharge current when a sample gas passes through the discharge region of a negative DC corona discharge. However, in actual use of such a detector, the sample gas would significantly contaminate the electrodes, and as a result, the long-term stability of the electrodes would be questionable.
A detector is sought that is suitable for portable use performing real-time measurements of contaminants in air and that avoids the problems of windows in PIDs, avoids the short operating times and explosion risks in FIDS, and avoids the stability problems associated with electrodes in conventional DIDs.